In a fiber optic network, it is desirable to produce all possible linear polarization states of light at the end of a length of fiber in order to perform polarization-based, PON testing. Typically, however, instead of being linearly polarized, the light at the end of an arbitrary length of fiber is elliptically polarized irrespective of the polarization state of the light at the beginning of the fiber. This is due to the xe2x80x9cbirefringencexe2x80x9d effect of the fiber. Birefringence causes light traveling through a fiber to split into a fast traveling wave component and a slow traveling wave component. When these two components are recombined at some point, their relative phase is different from their initial relative phase. This phase difference causes the polarization state of the light to change to a new elliptically polarized state. Therefor, methods and devices are needed to convert the elliptically polarized light at the end of a fiber to all possible linearly polarized states of light in order to facilitate polarization-based PON testing.
One method uses a transformation matrix and performs point-by-point calculations to derive input polarizations that will produce all possible linear polarization states. This method is time-consuming and inefficient, however.
It is therefor a desire of the present invention to provide methods and devices for producing all possible linear polarization states of light at the end of an arbitrary length of fiber which can be completed quickly.
In accordance with the present invention, there are provided methods and devices for producing all possible linear polarization states of light at the output of an optical fiber. According to the present invention, elliptically polarized light is detected at an output of a fiber and an output Poincare sphere is generated to represent an output polarization space for light at the output of the fiber. Various output polarization states are then generated such that the output polarization states form an arbitrary circle on the output Poincare sphere. Through steps which will be described in detail in the paragraphs that follow, the arbitrary circle is then manipulated to form a final circle which substantially coincides with an equator of the output Poincare sphere. The final circle represents all possible linear polarization states of light at the output of the fiber.
In an illustrative embodiment of the present invention, a novel system comprises a laser source, an input polarization controller and a set of wave plates to send light through an arbitrary length of fiber. Poincare spheres are used to represent polarization spaces for light waves. The polarization states of light at an output of the polarization controller and at an output end of the fiber are represented on a reference Poincare sphere and an output Poincare sphere, respectively. The output polarization state of light at the end of the fiber is displayed on a monitor as a point on the output Poincare sphere. The reference Poincare sphere need not be displayed, as it represents an intermediate polarization space of light in the system. Various components, which will later be described in detail, are then used to transform the polarization states of light so as to produce all possible linear polarization states of light at the end of the fiber. Another illustrative embodiment of the present invention employs a programmed device comprising a program and program code to perform substantially the same functions as the system.
In one step of a novel transformation method, an equatorial polarization angle xe2x80x9cxcex8xe2x80x9d, located in a plane containing an equator of the reference Poincare sphere, is continuously swept from 0xc2x0 to 360xc2x0 at the output of the polarization controller (i.e., input of the fiber). Sweeping xcex8 in such a manner causes the polarization states at the output of the fiber to trace an arbitrary circle on the output Poincare sphere. An input polarization of a source light input into the polarization controller is then adjusted such that, as xcex8 is swept from 0xc2x0 to 360xc2x0 at the output of the polarization controller, the arbitrary circle traced on the output Poincare sphere becomes smaller and smaller until it forms a reduced circle of a minimum radius. Next, the reduced circle is transformed so that it is centered at a pole of the output Poincare sphere. Finally, the input polarization of the source light is again adjusted so as to enlarge the transformed circle until the circle substantially coincides with the equator of the output Poincare sphere. The circle at the equator of the output Poincare sphere defines all possible linear output polarization states of light at the end of the fiber.
The present invention and its advantages can be best understood with reference to the drawings, detailed description of the invention and claims which follow.
Other objectives, features and advantages of the present invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.